VS2DT

Flow and Solute Transport in Variably Saturated, Single-Phase Flow in Porous Media


VS2DT OVERVIEW

VS2DT is a USGS program for flow and solute transport in variably saturated, single-phase flow in porous media. A finite-difference approximation is used to solve the advection-dispersion equation. Simulated regions include one-dimensional columns, two-dimensional vertical cross sections, and axially symmetric, three-dimensional cylinders. Program options include backward or centered approximations for both space and time derivatives, first-order decay, equilibrium adsorption (Freundlich or Langmuir) isotherms, and ion exchange. Nonlinear storage terms are linearized by an implicit Newton-Raphson method. Relative hydraulic conductivity is evaluated at cell boundaries using full upstream weighting, arithmetic mean or geometric mean. Saturated hydraulic conductivities are evaluated at cell boundaries using distance-weighted harmonic means.
The model has been modified to present pertinent data during execution. Five additional utility modules are included with this package. GREEN is the Green-Ampt analytical solution for infiltration. PRESS1D and CONCEN1D produce one-dimensional pressure and concentration profiles, respectively. MASSBAL produces mass balance rate component profiles. OBSERV produces volumetric moisture content, saturation and concentration time history data sets at each observation point.
VS2DT includes the source and executable codes, the five utility modules, and technical support.

DESCRIPTION

VS2DT uses a finite-difference approximation to the advection-dispersion equation.

Simulated regions can be:

Program options include:

VS2DT is a useful tool in studies of water quality, ground-water contamination, waste disposal, or ground-water recharge. The program is user oriented and easy to use.
VS2DT uses a finite-difference approximation to the advection-dispersion equation as well as the nonlinear water-flow equation (based on total hydraulic head).
The model can simulate problems in one, two (vertical cross section), or three dimensions (axially symmetric). The porous media may be heterogeneous and anisotropic, but principal directions must coincide with the coordinate axes. Nonlinear storage terms are linearized by an implicit Newton-Raphson method. Relative hydraulic conductivity is evaluated at cell boundaries using full upstream weighting, arithmetic mean or geometric mean. Saturated hydraulic conductivities are evaluated at cell boundaries using distance-weighted harmonic means. Moisture-characteristic curves may be represented by the Brooks-Corey, Haverkamp and van Genuchten models or interpolated from tabular data.
Boundary conditions for flow can take the form of fixed pressure heads, infiltration with ponding, evaporation from the soil surface, plant transpiration, or seepage faces.
Boundary conditions for solute transport include fixed solute concentration and fixed mass flux. Solute source/sink terms include first order decay, equilibrium partitioning to the solid phase (Langmuir or Freundlich isotherms), and ion exchange.
The design of VS2DT is modular and may be easily modified in order to apply the model to particular field, laboratory, or hypothetical problems.

SOLUTE TRANSPORT

The derivation of the equation in VS2DT is based on the conservation of mass and Fick's Law.
Three mechanisms affect the movement of solutes in the VS2DT model under variably saturated conditions:
VS2DT has been modified to present pertinent data (simulation time, iteration number, elapsed time) on the screen during execution.
Five additional utility modules have been developed and are included with the package: GREEN, MASSBAL, OBSERV, PRESS1D and CONCEN1D.
The VS2DT Package includes both source and executable codes for VS2DT and executable codes for the utility modules.

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